Page 1 of 8
KEY CONCEPT
Electromagnetic waves
have many uses.
BEFORE, you learned
NOW, you will learn
• EM waves transfer energy
through fields
• EM waves have measurable
properties
• EM waves interact with matter
• How EM waves differ from
one another
• How different types of EM
waves are used
VOCABULARY
EXPLORE Radio Waves
electromagnetic
spectrum p. 560
radio waves p. 562
microwaves p. 563
visible light p. 564
infrared light p. 564
ultraviolet light p. 565
x-rays p. 566
gamma rays p. 566
How can you make radio waves?
MATERIALS
PROCEDURE
1
Tape one end of one length of wire to one
end of the battery. Tape one end of the
second wire to the other end of the battery.
2 Wrap the loose end of one of the wires
tightly around the handle of the fork.
• two 25 cm lengths
of copper wire
• C or D battery
• electrical tape
• metal fork
• portable radio
3 Turn on the radio to the AM band and move
the selector past all stations until you reach static.
4 Hold the fork close to the radio. Gently pull
the free end of wire across the fork’s prongs.
WHAT DO YOU THINK?
• What happens when you stroke the prongs with
the wire?
• How does changing the position of the dial affect
the results?
EM waves have different frequencies.
reminder
Remember that frequency is
the number of waves that
pass a given point per second. The shorter the
wavelength, the higher
the frequency.
It might seem hard to believe that the same form of energy browns
your toast, brings you broadcast television, and makes the page you
are now reading visible. Yet EM waves make each of these events
possible. The various types of EM waves differ from each other in
their wavelengths and frequencies.
The frequency of an EM wave also determines its characteristics
and uses. Higher-frequency EM waves, with more electromagnetic
vibrations per second, have more energy. Lower-frequency EM
waves, with longer wavelengths, have less energy.
Chapter 17: Electromagnetic Waves 559
Page 2 of 8
The Electromagnetic Spectrum
The range of all EM frequencies is known as the electromagnetic
RESOURCE CENTER
CLASSZONE.COM
spectrum (SPEHK-truhm), or EM spectrum. The spectrum can be
Learn more about
represented by a diagram like the one below. On the left are the waves
the electromagnetic
with the longest wavelengths and the lowest frequencies and energies.
spectrum.
Toward the right, the wavelengths become shorter, and the frequencies
and energies become higher. The diagram also shows different parts
of the spectrum: radio waves, microwaves, infrared light, visible light,
ultraviolet light, x-rays, and gamma rays.
The EM spectrum is a smooth, gradual progression from the
lowest frequencies to the highest. Divisions between the different
parts of the spectrum are useful, but not exact. As you can see from
the diagram below, some of the sections overlap.
The Electromagnetic Spectrum
Frequency in Hertz (1 hertz = 1 wavelength/second)
104
105
106
107
108
109
1010
1011
1012
1013
Infrared Light
Radio Waves
Microwaves
The amount of infrared light an
object gives off depends on its
temperature. Above, different
colors indicate different amounts
of infrared light.
This woman is speaking on the radio.
Radio waves are used for radio and television broadcasts. They are also used for
cordless phones, garage door openers,
alarm systems, and baby monitors.
Not all astronomy involves visible light.
Telescopes like the one above pick up
microwaves from space. Microwaves are
also used for radar, cell phones, ovens,
and satellite communications.
560 Unit 4: Waves, Sound, and Light
Page 3 of 8
Measuring EM Waves
Because all EM waves move at the same speed in a vacuum, the
frequency of an EM wave can be determined from its wavelength.
EM wavelengths run from about 30 kilometers for the lowest-frequency
radio waves to trillionths of a centimeter for gamma rays. EM waves
travel so quickly that even those with the largest wavelengths have
very high frequencies. For example, a low-energy radio wave with a
wavelength of 30 kilometers has a frequency of 10,000 cycles per second.
SUPPORTING MAIN IDEAS
Write details that support
the main idea that EM
waves form a spectrum
based on frequency.
EM wave frequency is measured in hertz (Hz). One hertz equals
one cycle per second. The frequency of the 30-kilometer radio wave
mentioned above would be 10,000 Hz. Gamma ray frequencies reach
trillions of trillions of hertz.
check your reading
1014
1015
Why is wavelength all you need to know to calculate EM wave
frequency in a vacuum?
1016
1017
1018
1019
1020
1021
1022
1023
1024
Gamma Rays
Ultraviolet Light
X-Rays
Visible Light
Gamma rays can be
used to treat illnesses
and to create images
like this one of a person’s thyroid gland.
Visible light is the part of
the EM spectrum that can
be seen with the human
eye. This bird’s colors come
from different wavelengths.
X-rays are useful for showing
hard tissues inside the body, such
as bones. To make images like
the one above, x-ray images have
to be displayed using visible light.
The researcher in this photograph is
using ultraviolet light in the process
of DNA analysis. A chemical in the
samples gives off visible pink light
when ultraviolet rays are present.
Chapter 17: Electromagnetic Waves 561
Page 4 of 8
Radio waves and microwaves have long
wavelengths and low frequencies.
Radio waves are EM waves that have the longest wavelengths, the lowest
frequencies, and the lowest energies. Radio waves travel easily through
the atmosphere and many materials. People have developed numerous
technologies to take advantage of the properties of radio waves.
VOCABULARY
Make a frame game
diagram for radio waves
and the other types of EM
waves.
Radio Waves
Radio was the first technology to use EM waves for telecommunication,
which is communication over long distances. A radio transmitter
converts sound waves into radio waves and broadcasts them in different
directions. Radio receivers in many locations pick up the radio waves
and convert them back into sound waves.
1 Sound waves enter
the microphone and
are converted into
electrical impulses.
AM Signal
Information is encoded in
the signal by varying the
radio wave’s amplitude.
FM Signal
Information is encoded in
the signal by varying the
radio wave’s frequency.
2 The electrical impulses
are converted into radio
waves and broadcast
by the transmitter.
3 The radio waves
reach a radio receiver
and are converted
back into sound.
Different radio stations broadcast radio waves at different frequencies. To pick up a particular station, you have to tune your radio to
the frequency for that station. The numbers you see on the radio—
such as 670 or 99.5—are frequencies.
Simply transmitting EM waves at a certain frequency is not
enough to send music, words, or other meaningful sounds. To do
that, the radio transmitter must attach information about the sounds
to the radio signal. The transmitter attaches the information by
modulating—that is, changing—the waves slightly. Two common
ways of modulating radio waves are varying the amplitude of the
waves and varying the frequency of the waves. Amplitude modulation
is used for AM radio, and frequency modulation is used for FM radio.
You might be surprised to learn that broadcast television also uses
radio waves. The picture part of a TV signal is transmitted using an AM
signal. The sound part is transmitted using an FM signal.
check your reading
562 Unit 4: Waves, Sound, and Light
What two properties of EM waves are used to attach
information to radio signals?
Page 5 of 8
Microwaves
A type of EM waves called microwaves comes next on the EM
spectrum. Microwaves are EM waves with shorter wavelengths, higher
frequencies, and higher energy than other radio waves. Microwaves get
their name from the fact that their wavelengths are generally shorter
than those of radio waves. Two important technologies that use
microwaves are radar and cell phones.
reading tip
As you read about the
different categories of EM
waves, refer to the diagram
on pages 560 and 561.
The term radar stands for “radio detection and ranging.”
Radar came into wide use during World War II (1939–1945) as a way
of detecting aircraft and ships from a distance and estimating their
locations. Radar works by transmitting microwaves, receiving reflections of the waves from objects the waves strike, and converting these
patterns into visual images on a screen. Today, radar technology is
used to control air traffic at airports, analyze weather conditions, and
measure the speed of a moving vehicle.
Radar
Radar led to the invention of the microwave oven. The discovery
that microwaves could be used to cook food was made by accident
when microwaves melted a candy bar inside a researcher’s pocket.
A cell phone is actually a radio transmitter and receiver
that uses microwaves. Cell phones depend on an overlapping network
of cells, or areas of land several kilometers in diameter. Each cell has at
its center a tower that sends and receives microwave signals. The tower
connects cell phones inside the cell to each other or to the regular wirebased telephone system. These two connecting paths are shown below.
Cell Phones
microwave
tower
2
communication between two cell
phones in different cells
microwaves
wire-based system
(landlines)
1
communication
between two cell
phones in one cell
cell
Chapter 17: Electromagnetic Waves 563
Page 6 of 8
Infrared, visible, and ultraviolet light have
mid-range wavelengths and frequencies.
is the part of the EM spectrum that human eyes can see.
It lies between 1014 Hz and 1015 Hz. We perceive the longest wavelengths of visible light as red and the shortest as violet. This narrow
band is very small compared with the rest of the spectrum. In fact,
visible light is only about 1/100,000 of the complete EM spectrum.
The area below visible light and above microwaves is the infrared part
of the EM spectrum. Above visible light is the ultraviolet part of the
spectrum. You will read more about visible light in the next section.
Visible light
reading tip
Infrared means “below
red.” Ultraviolet means
“beyond violet.”
Infrared Light
The infrared light part of the spectrum consists of EM frequencies
between microwaves and visible light. Infrared radiation is the type
of EM wave most often associated with heat. Waves in this range are
sometimes called heat rays. Although you cannot see infrared radiation, you can feel it as warmth coming from the Sun, a fire, or a
radiator. Infrared lamps are used to provide warmth in bathrooms
and to keep food warm after it is cooked. Infrared rays also help
to cook food—for example, in a toaster or over charcoal.
The Electromagnetic Spectrum
How can you detect invisible light?
PROCEDURE
1
Find a place that has both bright sunlight and shade, such as a windowsill.
Place the white paper in the shade.
2 Using the marker, color the bulbs of the thermometers black. Place one ther-
mometer on the paper. After three minutes, record the temperature.
3 Position the prism so that it shines a bright color spectrum on the white
paper. Place the thermometers so that one bulb is in the blue area, one in
the red, and one just outside the red, as shown.
4 After five minutes, record the three temperatures.
WHAT DO YOU THINK?
• How did the temperature in the shade compare to the
temperature in the light and just outside of it?
• How might you explain the difference?
CHALLENGE How could you modify the experiment to find
the hottest location in the infrared range?
564 Unit 4: Waves, Sound, and Light
SKILL FOCUS
Drawing
conclusions
MATERIALS
•
•
•
•
white paper
black marker
3 thermometers
prism
TIME
30 minutes
Page 7 of 8
Some animals, such as pit viper snakes, can actually see
infrared light. Normally, human beings cannot see infrared
light. However, infrared scopes and cameras convert infrared
radiation into visible wavelengths. They do this by representing different levels of infrared radiation with different
colors of visible light. This technology can create useful
images of objects based on the objects’ temperatures.
check your reading
How do human beings perceive infrared radiation?
Ultraviolet Light
The ultraviolet light part of the EM spectrum consists of frequencies
above those of visible light and partially below those of x-rays.
Because ultraviolet (UV) light has higher frequencies than visible
light, it also carries more energy. The waves in this range can damage
your skin and eyes. Sunblock and UV-protection sunglasses are
designed to filter out these frequencies.
In this infrared image,
warmer areas appear red
and orange, while cooler
ones appear blue, green,
and purple.
Ultraviolet light has beneficial effects as well. Because it can damage
cells, UV light can be used to sterilize medical instruments and food by
killing harmful bacteria. In addition, UV light causes skin cells to produce vitamin D, which is essential to good health. Ultraviolet light can
also be used to treat skin problems and other medical conditions.
Like infrared light, ultraviolet light is visible to some animals. Bees
and other insects can see higher frequencies than people can. They see
nectar guides—marks that show where nectar is located—that people
cannot see with visible light. The photographs below show how one
flower might look to a person and to a bee.
This photograph shows the flower as it appears in
visible light.
This photograph shows the flower as it might appear
to a bee in ultraviolet light. Bees are able to see
nectar guides in the UV range.
Chapter 17: Electromagnetic Waves 565
Page 8 of 8
X-rays and gamma rays have short
wavelengths and high frequencies.
At the opposite end of the EM spectrum from radio waves are x-rays
and gamma rays. Both have very high frequencies and energies. X-rays
have frequencies from about 1016 Hz to 1021 Hz. Gamma rays have
frequencies from about 1019 Hz to more than 1024 Hz. Like other EM
waves, x-rays and gamma rays are produced
X-rays allow us to see inside the
by the Sun and by other stars. People have
body.
also developed technologies that use these
EM frequencies.
X-rays pass easily through the soft tissues
of the body, but many are absorbed by
denser matter such as bone. If photographic
film is placed behind the body and x-rays are
aimed at the film, only the x-rays that pass
through the body will expose the film. This
makes x-ray images useful for diagnosing
bone fractures and finding dense tumors.
But too much exposure to x-rays can damage
tissue. Even in small doses, repeated exposure
to x-rays can cause cancer over time. When you have your teeth x-rayed,
you usually wear a vest made out of lead for protection. Lead blocks
high-frequency radiation.
Gamma rays have the highest frequencies and energies of any EM
waves. Gamma rays are produced by some radioactive substances as well
as by the Sun and other stars. Gamma rays can penetrate the soft and
the hard tissues of the body, killing normal cells and causing cancer
cells to develop. If carefully controlled, this destructive power can be
beneficial. Doctors can also use gamma rays to kill cancer cells and
fight tumors.
KEY CONCEPTS
CRITICAL THINKING
1. What two properties of EM
waves change from one end of
the EM spectrum to the other?
4. Infer Why do you think
remote controls for TVs, VCRs,
and stereos use infrared light
rather than ultraviolet light?
2. Describe two uses for
microwave radiation.
3. How are EM waves used in
dentistry and medicine?
566 Unit 4: Waves, Sound, and Light
5. Apply For a camera to make
images of where heat is
escaping from a building in
winter, what type of EM wave
would it need to record?
CHALLENGE
6. Synthesize When a person
in a car is talking on a cell
phone, and the car moves
from one cell to another, the
conversation continues without
interruption. How might this
be possible?